This invention relates to silicon devices and more particularly to a high-temperature metallization on ring or mesa silicon devices for high-power, wide-pulse, continuous-wave microwave applications.
Heretofore junction impact avalanche transit time (IMPATT) diodes and trapped plasma avalanche triggered transit (TRAPATT) diodes have been developed which oscillate or amplify at microwave frequencies when mounted in an appropriate circuit and reverse-biased with sufficient current. Trapped plasma avalanche triggered transit (TRAPATT) diodes are two-terminal avalanche devices with single p.sup.+ -n-n.sup.+ or p.sup.+ -p-n.sup.+ structures which operate at higher threshold current, higher efficiency, higher power output, and lower frequency than IMPATT diodes. High efficiency of TRAPATT diodes is due to their ability to rapidly switch between a very high resistance state and a very low resistance state. The high-power densities at which TRAPATTs must operate produce severe electrical and thermal stresses. Special requirements must be imposed upon material (silicon) quality, device design, fabrication procedure, and heat sink configuration when TRAPATTs are operated CW or at wide pulse-widths.
Prior-art TRAPATTs have been fabricated using silicon epitaxial material upon which a gold layer has been deposited over a chromium layer. The diodes are then formed by well-known photolithographic etch processes and a gold mesa or ring is electroplated on the gold layer to provide a supporting base for securing electrical contacts. The opposite side of the epitaxial silicon material has a chromium layer and a gold layer which is secured onto a metallized diamond heat sink. Such a TRAPATT diode cannot withstand temperatures higher than 370.degree. C. because the chromium is no barrier to the diffusion of gold into the silicon, and gold and silicon form a liquid phase at 370.degree. C. Instead of chromiumgold, titanium-tungsten/gold may be used. These may be used as high temperature metallization for planar devices but cannot be used where silicon etching is required because (titanium-tungsten) dissolves in the etch. Prior art TRAPATT diodes have been set forth in an article "TRAPATT's High Power Devices for Wide Pulse and CW Microwave Applications," by K Reed Gleason et al, IEEE Transactions on Parts Hybrids and Packaging, Vol. PHP-13, No. 4, pp. 344-348, December 1977.